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Faulty replication systems lead to the mutations in the genome.
Lead to different surface proteins/antigens.
It's the surface proteins that bind to cells (and do other things)
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/surface-proteinsIf the virus is recognised instantly by the host immune system as "intruder" it's packaged up and responded to. If it manages to hide away for a bit, it manages to replicate and cause damage.
Microorganisms have lots of really interesting strategies. A famous one is the malarial parasite Plasmodium (and other apicomplexa like Toxoplasma do this), where they encyst and stay hidden. And then when you're run down they have a tendency to reemerge and make you ill again.
As in the faulty replication systems give rise to the mutations that enable the leap to humans?